celestory insulation

[sunspot]

I've just started replacing six celestory windows. I'm adding R6 Roxul Comfort Board to the exterior under rain screen and vinyl siding.
My question for the group relates to the interior.
The drywall is nasty. There is some poorly installed vapor barrier behind it. I'm wondering about covering the existing drywall with some sort of painted foam to increase insulation value and give me a lightweight, easily installed, interior finish (I'd hide the joints with trim). No fire resistance I realize. I'm also wondering about tipping the foam wall covering on an angle to provide room for additional insulation at the wall plate/rafter interface which I assume is currently a source of heat loss.
If this idea is madness please stop me now before I get very far.
Thanks,